1) Field of the Invention
The present invention relates to a developing device used in an electrophotographic field, wherein an electrostatic latent image is visually developed by using a one-component developer.
2) Description of the Related Art
As is well known, an electrophotographic printer carries out the processes of: producing a uniform distribution of electrical charges on a surface of an electrostatic latent image carrying body such as an electrophotographic photoreceptor; forming an electrostatic latent image on the electrically charged surface of the electrophotographic photoreceptor by optically writing an image thereon by using a laser beam scanner, an LED (light emitting diode) array, an LCS (liquid crystal shutter) array or the like; visually developing the electrostatic latent image with a developer, i.e., toner, which is electrically charged to be electrostatically adhered to the electrostatic latent image zone; electrostatically transferring the developed visible image to a paper; and fixing the transferred image on the paper. Typically, the electrophotographic photoreceptor is formed as a photosensitive drum having a cylindrical conductive substrate and a photoconductive insulating film bonded to a cylindrical surface thereof.
In the developing process, a two-component developer composed of a toner component (colored fine synthetic resin particles) and a magnetic component (magnetic fine carriers) is widely used, as it enables a stable development of the latent image. Note, typically the toner particles have an average diameter of about 10 .mu.m, and the magnetic fine carriers have a diameter ten times larger than the average diameter of the toner particles. Usually, a developing device using the two-component developer includes a vessel for holding the two-component developer, wherein the developer is agitated by an agitator provided therein. This agitation causes the toner particles and the magnetic carriers to be subjected to triboelectrification, whereby the toner particles are electrostatically adhered to the magnetic carriers. The developing device also includes a magnetic roller provided within the vessel as a developing roller, in such a manner that a portion of the magnetic roller is exposed therefrom and faces the surface of the photosensitive drum. The magnetic carriers with the toner particles are magnetically adhered to the surface of the magnetic roller to form a magnetic brush therearound, and by rotating the magnetic roller carrying the magnetic brush, the toner particles are brought to the surface of the photosensitive drum for the development of the electrostatic latent image formed thereon.
In this developing device, a ratio between the toner and magnetic components of the developer body held in the vessel must fall within a predetermined range, to continuously maintain a stable development process. Accordingly, the developing device is provided with a toner supplier from which a toner component is supplied to the two-component developer held in the vessel, to supplement the toner component as it is consumed during the development process, whereby the component ratio of the two-component developer held by the vessel is kept within the predetermined range. This use of a two-component developer is advantageous in that a stable development process is obtained thereby, but the developing device per se has the disadvantages of a cumbersome control of a suitable component ratio of the two-component developer, and an inability to reduce the size of the developing device due to the need to incorporate the toner supplier therein.
A one-component developer is also known in this field, and a developing device using the same does not suffer from the above-mentioned disadvantages of the developing device using the two-component developer, because the one-component developer is composed of only a toner component (colored fine synthetic resin particles.) Two types of the one-component developer are known; a magnetic type and a non-magnetic type. A developing device using the magnetic type one-component developer can be constructed in substantially the same manner as that using the two-component developer. Namely, the magnetic type one-component developer also can be brought to the surface of the photosensitive drum by a rotating magnetic roller as in the developing device using the two-component developer. The magnetic type one-component developer is suitable for achromatic color (black) printing, but is not suitable for chromatic color printing. This is because each of the toner particles of which the magnetic type one-component developer is composed includes fine magnetic powders having a dark color. In particular, the chromatic color printing obtained from the magnetic type one-component developer appears dark and dull, due to the fine magnetic powders included therein. Conversely, the non-magnetic type one-component developer is particularly suitable for chromatic color printing because it does not include a substance having a dark color, but the non-magnetic type one-component developer cannot be brought to the surface of the photosensitive drum by the magnetic roller as mentioned above.
A developing device using the non-magnetic type one-component developer is also known, as disclosed in U.S. Pat. Nos. 3,152,012 and 3,754,963. This developing device includes a vessel for holding the non-magnetic type one-component developer, and a conductive elastic solid roller provided within the vessel as a developing roller in such a manner that a portion of the elastic roller is exposed therefrom and can be pressed against the surface of the photosensitive drum. The conductive elastic solid developing roller may be formed of a conductive silicone rubber material or a conductive polyurethane rubber material, as disclosed in Japanese Examined Patent Publication (Kokoku) No. 60-12627 and Japanese Unexamined Patent Publications (Kokai) No. 62-118372 and No. 63-189876. When the conductive solid rubber roller is rotated within the body of the non-magnetic type one-component developer held by the vessel, the toner particles composing the non-magnetic type one-component developer are frictionally entrained by the surface of the conductive solid rubber developing roller to form a developer layer therearound, whereby the toner particles can be brought to the surface of the photosensitive drum for the development of the electrostatic latent image formed thereon. The developing device further includes a blade member engaged with the surface of the developing roller, to uniformly regulate a thickness of the developer layer formed therearound so that an even development of the latent image can be carried out. The blade member also serves to electrically charge the toner particles by a triboelectrification therebetween. In this developing device, the development process is carried out in such a manner that, at the area of contact between the photosensitive drum and the conductive solid rubber developing roller carrying the developer layer, the charged toner particles are electrostatically attracted and adhered to the latent image due to a bias voltage applied to the conductive solid rubber developing roller.
To achieve a proper development of the latent image by the developing rubber roller, an elasticity or hardness of the developing roller is an important parameter, because the development quality and the development toner density are greatly affected by a contact or nip width between the photosensitive drum and the solid rubber developing roller pressed thereagainst. Namely, the developing roller must be pressed against the photosensitive drum so that a given nip width by which a proper development is obtained is established therebetween. The conductive silicone or polyurethane solid rubber developing roller has a relatively high hardness. For example, when measured by an Asker C-type hardness meter, the solid rubber developing roller showed an Asker C-hardness of about 58.degree.. Accordingly, the solid rubber developing roller must be pressed against the photosensitive drum with a relatively high pressure to obtain the required nip width therebetween, but the higher the pressure exerted upon the photosensitive drum by the developing roller, the greater the premature wear of the drum.
Japanese Unexamined Patent Publication No. 63-100482 discloses a developing roller comprising a sponge roller element covered with a silicone solid rubber layer, whereby a penetration of the toner particles into the sponge roller element is prevented. This developing sponge roller is softer than the solid rubber developing roller, and thus the required nip width between the developing roller and the photosensitive drum can be obtained without exerting a high pressure upon the drum. Nevertheless, the production of the sponge developing roller is costly due to the complex construction thereof. Also, this developing roller has a low reliability in operation because the silicone solid layer can be separated from the sponge roller element.
Furthermore, the developing device using the non-magnetic type one-component developer must be constituted in such a manner that the toner particles can be given a charge distribution that will produce a proper development of a latent image, since if this is not ensured, an electrophotographic fog may appear during the development process and the developer be wastefully consumed for the reasons stated hereinafter in detail. Also, the developing device is preferably constituted in such a manner that the charge distribution ensuring a proper development of a latent image is stably obtained without being affected by variations of the temperature and air moisture content.